1. Field of the Invention
The present invention relates to a process for manufacturing a superconducting material, more particularly it relates to an after-treatment of a sintered oxide type superconductor.
2. Description of the Related Art
The superconductivity is a phenomenon which is understood as a phenomenon of phase change of electrons under which the electrical resistance become zero and the perfect diamagnetism is observed. If the superconductor is applied to the present power cables which loses 7% of energy, electric current of a very high current density can be delivered without any loss of power. The superconductor can be utilized also on a form of a magnetic coil in the field of power electric applications such as MHD power generation, power transmission, electric power reservation or the like; in the field of transportation such as magnetic levitation trains or magnetically propelling ships; a high sensitive sensors or detectors for sensing very weak magnetic field, microwave, radiant ray or the like, in the medical field such as high-energy beam radiation unit; in the field of science such as NMR or high-energy physics; or in the field of fusion power generation.
In addition to the abovementioned power electric applications, the superconducting materials can be used in the field of electronics, for example, as a Josephson device which is an indispensable switching device for realizing a high-speed computer which consumes very reduced power.
However, the critical temperature could not exceed 23.2K of Nb.sub.3 Ge which was the the highest Tc heretofore for all studies for the past ten years.
Possibility of existence of a new type of superconducting materials having much higher Tc was revealed by Bednorz and Muller who discovered a new oxide type superconductor in 1986 [Z. Phys. B64 (1986) 189].
It had been known that certain ceramics material of compound oxides exhibit the property of superconductivity. For example, U.S. Pat. No. 3,932,315 discloses Ba-Pb-Bi type compound oxide which shows superconductivity and Japanese patent laid-open No. 60-173,885 discloses that Ba-Bi type compound oxides also show superconductivity. These type superconductors, however, possess a rather lower transition temperature of about 10K and hence usage of liquidized helium (boiling point of 4.2K) as cryogen is indispensable to realize superconductivity.
This new oxide type superconducting material discovered by Bednorz and Muller is [La, Ba]2CuO.sub.4 which is called as the K.sub.2 NiF.sub.4 -type oxide having a crystal structure which is similar to known perovskite type oxide. The K.sub.2 NiF.sub.4 -type oxides show such higher Tc as 30K which are extremely higher than the known superconducting materials.
It was also reported in the news parer that C. W. Chu et al discovered in the United States of America another type of superconducting material having the critical temperature of in the order of 90K in February 1987, and hence possibility of existence of high-temperature superconductors have burst on the scene.
The abovementioned new type superconducting materials are usually produced in a form of bulky mass by sintering technique and in a form of a thin film by physical vapour deposition technique. In most case, the superconducting material produced by these techniques must be further subjected to thermal-treatment in an oxygen containing atmosphere in order to improve their property as superconductivity.
However, even after the thermal-treatment, the superconducting property of the oxide type superconductors is not stable and also the discrepancy (.DELTA.T) between the onset temperature and the critical temperature can not be decreased.
Therefore, an object of the present invention is to overcome the abovementioned problems of the conventional technique and to provide a novel process for producing an improved superconducting material which has high Tc and high stability.